1. Field of the Invention
The present invention relates to porous beads. More particularly, the present invention is concerned with substantially spherical porous beads of an aromatic polyether ketone resin, which have an average particle diameter of 50 to 5,000 xcexcm and a porosity of 40 to 99%. The porous beads of the present invention have various excellent properties, such as high resistance to heat and chemicals, and a high capability to prevent dust generation, and by virtue of these excellent properties, the porous beads of the present invention can be advantageously used as various carriers for separation of a substance (such as a carrier for chromatography and an enzyme-immobilized carrier) and as filter materials for use in the field of water treatment. Especially, the porous beads of the present invention have not only a high heat resistance, but also a high resistance to hydrolysis, a high capability to prevent dust generation and a high porosity, and exhibit only a small pressure loss and, hence, the porous beads of the present invention can be advantageously used as filter materials for treating a heater drainage and the like from a nuclear power plant or a thermal power plant, in which the treatment is conducted by contacting the filter material with the heater drainage to remove impurities, such as ferric components, from the drainage. Further, the present invention is also concerned with a method for producing the above-mentioned porous beads having high resistance to heat and chemicals, and a high capability to prevent dust generation. By the use of the method of the present invention, it becomes possible to produce porous beads having a desired average particle diameter and a desired porosity, with ease.
2. Prior Art
A polyether ketone resin is a resin having a high chemical stability, which has, for example, high resistance to heat and solvents. One example of the aromatic polyether ketone resin is a poly(oxy-1,4-phenylene oxy-1,4-phenylene-carbonyl-1,4-phenylene) which is generally known as poly(ether ether ketone) or PEEK. PEEK has a high melting point (334xc2x0 C.) and a high glass transition temperature (143xc2x0 C.), and is a thermoplastic, crystalline polymer having excellent properties, such as a high heat resistance, a high chemical stability and a high resistance to hydrolysis. The high chemical stability of PEEK is an advantageous and useful property for a membrane and, hence, it has been attempted to use PEEK for production of membrane filters (in the form of porous hollow fibers, porous flat membranes and the like) since a long time ago. However, when PEEK is used as a material for producing porous materials, such as porous hollow fibers and sheet-type separation membranes, by wet process or thermally induced phase separation process, the types of solvents and the temperatures, which both can be employed for the production of the porous materials, are limited due to the intrinsic high chemical stability of PEEK. Therefore, it has been difficult to produce porous materials from PEEK.
Unexamined Japanese Patent Application Laid-Open Specification No. 4-170445 discloses porous micro-particles of an aromatic polyether ketone resin. The porous microparticles disclosed in this patent document have a small particle diameter, namely only 1 xcexcm to 40 xcexcm, so that the use of such microparticles as a separation carrier or a filter material is accompanied by the following problems.
When particulate porous materials (porous partiales) are used as a separation carrier or a filter material, in general, a packed bed of the porous particles is provided in a column (or a tower) having, at a lower portion thereof, a porous support for retaining the porous particles, and a liquid is passed or filtered through the packed bed. When the particle diameter of the porous particles is small, the pressure loss becomes large, so that it is difficult to prepare a packed bed having a satisfactory thickness. In addition, when the pore diameter of the porous particles is small, there is a necessity to decrease the pore diameter of the porous support and in turn the small pore diameter of the porous support causes an increase in pressure loss.
In order to increase the throughput of a separation or filtration apparatus, it is necessary to increase the number of columns (towers) and/or the surface area of the porous support. A conceivable measure for increasing the surface area of the porous support is to use, as the porous support, a hollow fiber filter, a pleats filter or the like (which has a large surface area per unit volume thereof), which has formed thereon a coating (formed by deposition) of the porous particles. However, when such hollow fibers having a coating of porous particles are used in a filter module for use in an application field where the heat stability and chemical stability of PEEK are important, for example, in a filter module for removing impurities, such as ferric components, from hot water (such as a heater drainage and the like from a nuclear power plant or a thermal power plant) or for the treatment of polar solvents, the use of the filter module encounters problems of poor heat resistance and solvent resistance of the materials (other than PEEK) used for producing the filter module. Specifically, for example, in a hollow fiber module prepared by fluid-tightly fixing a bundle of hollow fibers in a module casing, the low heat resistance and solvent resistance of an adhesive used for fixing the follow fibers to the module casing are likely to become serious problems in the treatment of the above-mentioned hot water.
As examples of materials for producing the filter modules, which materials need to exhibit high resistance to heat and solvents, there can be mentioned a sintered filter made of stainless steel and a stainless steel filter. Even when these materials are used, the increase in the surface area for increasing the throughput of the filter modules is accompanied by problems that the construction of the column (or tower) becomes complicated and that the whole apparatus, including the column (or tower), becomes large, thereby increasing the cost of equipment.
Further, Unexamined Japanese Patent Application Laid-Open Specification No. 4-170445 discloses porous microparticles of an aromatic polyether ketone resin. The porous microparticles disclosed in this patent document have thin scale-like or thread-like projections (hereinfater, frequently referred to simply as xe2x80x9cprojectionsxe2x80x9d) of the polymer constituting the particles. When such particles get in contact with each other, the projections come off the particles and generate dust. Therefore, when the porous microparticles disclosed in this patent document are used, for example, for filtration, the filtration will be accompanied by a pressure loss which is caused not only due to the small particle diameter of the microparticles, but also by the dust generated from the projections which have come off the microparticles.
In addition, when such microparticles (beads) which are likely to generate dust are packed in a column and used for filtration, the filtration will be accompanied by not only a large pressure loss, but also a serious problem that the dust itself becomes a waste which harmfully affects the filtration operation. Specifically, most of the projections which have come off the particles are accumulated on the porous support of the column, while some of the projections pass through the porous support. In general, the pore diameter of the porous support is designed taking into consideration the particle diameter of the particles used as a separation carrier and, thus, the projections (having a size far smaller than that of the particles) which have come off the particles easily pass through the pores of the porous support. The dust (projections) which has passed through the porous support moves through various conduits and causes clogging in other equipment connected to the filtration apparatus. For example, the dust is caused to adhere to the inner walls of the conduits of the heat exchanger, thereby forming scale-like deposits, which cause problems, such as a marked lowering of the heat conductivity.
In this situation, the present inventors have made extensive and intensive studies with a view toward developing porous beads which not only have excellent resistance to heat and chemicals, but also exhibit only a small pressure loss, so that the beads can be used for the filtration of a large volume of hot water or for the adsorption of impurities contained in hot water. As a result, it has unexpectedly been found that, when porous beads are produced by a method mentioned below, the produced porous beads not only have a large average particle diameter and a high porosity, and exhibit a pressure loss which is very small, but also have high resistance to heat, chemicals and hydrolysis. The method comprises: mixing, while heating, an aromatic polyether ketone resin with a solvent capable of dissolving the resin to thereby obtain a resin solution; dispersing the resin solution in a liquid dispersion medium which is incapable of dissolving the resin and is non-miscible with the solvent, to thereby obtain a dispersion having the resin solution dispersed therein in the form of globules; cooling the dispersion while maintaining the dispersed form of the resin solution in the liquid dispersion medium, to thereby solidify the resin in the resin solution; and separating the solidified resin from a mixture of the solvent and the liquid dispersion medium. Further, with respect to the porous beads produced by the above-mentioned method, the present inventors have found that the porous beads have continuous pores in a three-dimensional network, that porous beads are smooth and, hence, are unlikely to generate dust, and that the porous beads are skinless.
The present invention has been completed, based on the above-mentioned novel findings.
Accordingly, it is an object of the present invention to provide porous beads which can be used as a filter material or adsorbent which not only has high resistance to heat, chemicals and hydrolysis, but also has an excellent capability to prevent dust generation and high mechanical strength, and exhibits a pressure loss which is very small.
It is another object of the present invention to provide a method for producing porous beads, which is advantageous in that the particle diameter and porosity of the porous beads can be easily controlled.